Left bundle branch block-Innocent bystander, silent menace, or both.

Left bundle branch block (LBBB) causes immediate electrical and mechanical dyssynchrony of the left ventricle (LV) and gradual structural damages in the Purkinje cells and myocardium. Mechanical dyssynchrony reduces the LV ejection fraction (EF) instantly, but only to ≈55% in an otherwise normal heart. Because of the heart's in-built functional redundancy, a patient with LBBB does not always notice the heart's reduced efficiency straightaway. After a variable period of time (which could be from days to decades), the patient may become symptomatic with heart failure (HF), which classifies as HF with preserved EF ≥50% (HFpEF). The LVEF drops further because of continuous adverse remodeling and inefficient cardiac contraction. The patient transits to HF with moderately reduced EF 35%-50% (HFmrEF) and then reduced EF ≤35% (HFrEF) over 5-21 years. Cardiac resynchronization therapy (CRT) is currently only indicated in guidelines for HFrEF and LBBB. LBBB shortens the median survival of patients with HFmrEF by 5.5 years. Randomized controlled trials have shown that CRT improves echocardiographic indices for HFmrEF with LBBB. CRT in HFpEF with LBBB is a promising but underexplored/underused therapy. There have been anecdotal reports that CRT produced symptom relief in patients debilitated by HFpEF with LBBB, who constitute ≈6% of all patients with HF and an adequate pool of potential randomized controlled trial participants. Conduction system pacing in the form of left bundle branch area pacing is an emerging pacing strategy that might reverse and forestall the deleterious effects of LBBB.